1. Field of the invention
The invention concerns a regulator system for a transistorized inverter generator feeding an inductively coupled load.
2. Description of the prior art
Induction heatint entails creating an alternating magnetic field in a conductive material, this magnetic field inducing Eddy currents in the material which dissipate energy within the latter by virtue of the Joule effect.
In principle, the energy dissipated within the material and the power transmitted per unit volume can by very high. The energy is dissipated at the surface of the part subjected to the induction phenomenon, the thickness to which energy penetrates into the material from which it ismade depending on the magnetic characteristics of the material and on the frequency of the alternating magnetic field.
In all applications the load as "seen" by the induction generator used may be represented by a series or parallel RLC circuit, the inductor having an inductive and resistive impedance, impedance matching being achieved by means of transformers, the capacitors thus operating at an acceptable voltage.
In the applications previously mentioned, the impedance or load as seen by the generator is constantly varying, in particular on passing through the Curie point for the magnetic material and transfer into or movement within the inductor or induction furnance of the parts to be treated.
The generators employed consist of AC-DC-AC converters and feed the series resonant RLC circuit. The generators correspond, for example, to a bridge or half-bridge circuit in which the RLC load is connected in series with the output of an inverter, constituting a controlled generator, through the intermediary of at least two zero-voltage switches. The switches comprise switching elements, the switchng terminals of the latter being connected in parallel with a diode and a capacitor. Generators of this kind available through ordinary commercial channels and the corresponding waveforms are shown in FIGS. 1a through 1d in the non-limiting case where the switches consist of "dual" thyristors, this type of component being well known in the current state of the art and shown in FIG. 1a in particular.
FIG. 1b shows a half-bridge inverter in which "dual" thyristors are used as the switches 1 and 2.
With this type of generator, the fact of working with a voltage source and of having a series load requires in practice that at the operating frequency the frequency of the control signal U.sub.C is at all times higher than the resonant frequency of the load consisting of the series resonant RLC circuit.
The power transmitted to the load by the generator depends on the amplitude of the supply voltage delivered by the generator, the voltage U applied to the inductor and the current I.sub.RLC flowing through the load, together with the phase difference between the last two of these parameters. For a given amplitude of the supply voltage the power transmitted is maximal when the operating frequency is equal to the resonant frequency of the load. This is a limiting case which cannot be achieved because of the highly variable operating conditions as previously mentioned.
An object of the invention is to overcome the aforementioned disadvantage by using a regulator system for a switching mode inverter generator feeding an inductively coupled load in which maximal transmission of power to the load is achieved at all times by continously slaving the frequency of the supply voltage to a frequency near the resonant frequency of the load.
Another object of the switching mode inverter regulator system in accordance with the invention is to regulate the power transmitted inductively to the load.